1. Field of the Invention
The present invention relates to a refractive index variable element that can significantly vary refractive index upon irradiation with light.
2. Description of the Related Art
In an optical or electronic function device or system which uses light as an information medium, it is absolutely necessary to control the refractive index of a component material or device. This is because the propagation characteristics of light are governed by the refractive index. Therefore, it is important to design a device so as to establish prescribed refractive index distribution, to arrange a material with a prescribed refractive index in the device, or to vary the refractive index of the device, not only in an optical waveguide and an optical fiber but also in an optical switching device and an optical recording device.
Known methods for significantly varying the refractive index include (1) Stark shift, (2) Franz-Keldysh effect, (3) Pockels effect, (4) Kerr effect, (5) orientation variation, (6) level splitting by magnetic field, (7) Cotton-Mouton effect, (8) optical Stark shift, (9) absorption saturation, (10) electromagnetically induced transparency (EIT), (11) photoisomerization, (12) structural change by light irradiation, (13) photoionization, (14) piezoreflection effect, (15) thermal band shift, (16) thermal isomerization, and (17) thermally-induced structural change. Techniques of varying the refractive index through the Pockels effect are disclosed in, for example, Jpn. Pat. Appln. KOKAI No. 2002-217488, Jpn. Pat. Appln. KOKAI No. 11-223701, and Jpn. Pat. Appln. KOKAI No. 5-289123.
The refractive index can be represented by a complex number in which a real part thereof denotes the refractive index in the narrow sense and an imaginary part thereof denotes absorption. In the mechanisms for the refractive index variation cited above, the variation in the real part of the complex refractive index is large in the absorption region and the absorption edge, but is small, i.e., not larger than 1%, in the non-absorption region. Also, an optical function device utilizing variation in absorbance, such as a light-absorption type optical switch, is being studied. However, such absorption implies that the intensity of the light beam carrying the information is lowered. Thus, it is desirable that the real part of the complex refractive index can be greatly varied in a non-absorption wavelength region. Among the refractive index variable materials, liquid crystal exhibits an exceptionally large variation not smaller than 10% in the real part of the complex refractive index in the non-absorption wavelength region. This is because the refractive index variation of liquid crystal is brought about by the variation in orientation, not by the electronic polarizability variation. Taking application of a-refractive index variable material to an optical function device into consideration, however, a liquid refractive index variable material such as liquid crystal can only be applicable to significantly limited fields.